Multiple-output switching power source apparatus

ABSTRACT

A multiple-output switching power source apparatus has a series resonant circuit connected in parallel with a switch Q 2  and including a primary winding and a current resonant capacitor, a first rectifying-smoothing circuit rectifying and smoothing a voltage of a secondary winding in an ON period of the switch to provide a voltage Vo 1 , a series resonant circuit connected in parallel with the switch and including a primary winding and a current resonant circuit, a second rectifying-smoothing circuit rectifying and smoothing a voltage of a secondary winding in the ON period of the switch Q 2  to provide a voltage Vo 2 , and a control circuit controlling an ON period of a switch Q 1  according to the output voltage Vo 1  and the ON period of the switch Q 2  according to the voltage Vo 2  and limit the ON period of the switch Q 1  if the voltage Vo 2  exceeds a predetermined voltage.

TECHNICAL FIELD

The present invention relates to a multiple-output switching powersource apparatus having a plurality of outputs.

BACKGROUND TECHNOLOGY

FIG. 1 is a circuit diagram illustrating a configuration of aresonant-type multiple-output switching power source apparatus accordingto a related art. In this multiple-output switching power sourceapparatus, the primary side of a transformer T1 includes a full-waverectifying circuit 2 to rectify an AC voltage from a commercial powersource 1, a smoothing capacitor C3 connected between output terminals ofthe full-wave rectifying circuit 2, to smooth an output from thefull-wave rectifying circuit 2, first and second switching elements Q1and Q2 made of, for example, MOSFETs connected in series between bothends of the smoothing capacitor C3 and receiving a voltage across thesmoothing capacitor C3 as a DC input voltage Vin, a control circuit 10to control ON/OFF of the first and second switching elements Q1 and Q2,a voltage resonant capacitor Crv connected in parallel with the secondswitching element Q2, and a series resonant circuit connected to bothends of the voltage resonant capacitor Crv.

The series resonant circuit has a primary winding P1 (the number ofturns of N1) of the transformer T1, a reactor Lr, and a current resonantcapacitor Cri that are connected in series. The reactor Lr is, forexample, a leakage inductance between the primary and secondary sides ofthe transformer T1.

The secondary side of the transformer T1 includes a firstrectifying-smoothing circuit connected to a first secondary winding S1(the number of turns of N2) wound to generate a voltage whose phase isopposite to the phase of a voltage generated by the primary winding P1of the transformer T1 and a second rectifying-smoothing circuitconnected to a second secondary winding S2 (the number of turns of N3)wound to generate a voltage whose phase is opposite to the phase of thevoltage generated by the primary winding P1 of the transformer T1.

The first rectifying-smoothing circuit has a diode D1 and a smoothingcapacitor C1, to rectify and smooth a voltage induced by the firstsecondary winding S1 of the transformer T1 and output a first outputvoltage Vo1 from a first output terminal. The secondrectifying-smoothing circuit has a diode D2 and a smoothing capacitorC2, to rectify and smooth a voltage induced by the second secondarywinding S2 of the transformer T1 and output a second output voltage V02from a second output terminal.

The multiple-output switching power source apparatus also has a feedbackcircuit 5 to feed a signal corresponding to a voltage generated on thesecondary side of the transformer T1 back to the primary side. An inputside of the feedback circuit 5 is connected to the first outputterminal. The feedback circuit 5 compares a voltage across the smoothingcapacitor C1 with a predetermined reference voltage and feeds an errorvoltage as a voltage error signal back to the control circuit 10 on theprimary side.

According to the voltage error signal from the feedback circuit 5, thecontrol circuit 10 alternately turns on/off the first and secondswitching elements Q1 and Q2 to conduct PWM control in such a way as tokeep the first output voltage Vo1 constant. Each gate of the first andsecond switching elements Q1 and Q2 receives, as a control signal, avoltage involving a dead time of about several hundreds of nanoseconds.This enables the first and second switching elements Q1 and Q2 toalternately turn on/off without the ON periods of the first and secondswitching elements Q1 and Q2 overlapping.

Operation of the multiple-output switching power source apparatusaccording to the related art having the above-mentioned configurationwill be explained with reference to waveforms illustrated in FIG. 2.

In FIG. 2, VQ2 ds is a drain-source voltage of the second switchingelement Q2, IQ1 is a current passing through a drain of the firstswitching element Q1, IQ2 is a current passing through a drain of thesecond switching element Q2, Icri is a current passing through thecurrent resonant capacitor Cri, Vcri is a voltage across the currentresonant capacitor Cri, ID1 is a current passing through the diode D1,VN2 is a voltage across the first secondary winding S1, and ID2 is acurrent passing through the diode D2.

The first output voltage Vo1 is controlled by the control circuit 10that receives a voltage error signal fed back to the primary side fromthe first rectifying-smoothing circuit through the feedback circuit 5and conducts the PWM control on the first switching element Q1. Asmentioned above, the first and second switching elements Q1 and Q2 arealternately turned on/off according to control signals from the controlcircuit 10 with a dead time of about several hundreds of nanoseconds.

In an ON period (for example, from time t11 to t12) of the firstswitching element Q1, the current resonant capacitor Cri accumulatesenergy through an exciting inductance of the primary winding P1 of thetransformer T1 and the reactor Lr (leakage inductance between theprimary and secondary sides of the transformer T1).

In an ON period (for example, from time t12 to t14) of the secondswitching element Q2, the energy accumulated in the current resonantcapacitor Cri causes the reactor Lr and current resonant capacitor Crito pass a resonant current and send energy to the secondary side. Theexciting energy of the exciting inductance of the primary winding P1 isreset.

More precisely, in the ON period of the second switching element Q2, theprimary winding P1 receives a voltage that is obtained by dividing thevoltage Vcri across the current resonant capacitor Cri by the excitinginductance of the primary winding P1 and the reactor Lr. When thevoltage applied to the primary winding P1 reaches a level of(Vo1+Vf)×N1/N2, the voltage is clamped and the current resonantcapacitor Cri and reactor Lr pass a resonant current and send energy tothe secondary side. This results in passing the current ID1 through thediode D1. When the voltage of the primary winding P1 is smaller than thelevel of (Vo1+Vf)×N1/N2, no energy is sent to the secondary side of thetransformer T1 and the exciting inductance of the primary winding P1 ofthe transformer T1, the reactor Lr, and the current resonant capacitorCri produce a resonant operation only on the primary side.

The ON period of the second switching element Q2 is determined by the ONperiod of the first switching element Q1 under a fixed frequency, or isan optional constant period. The ON period of the first switchingelement Q1 may be changed to change duty ratios of the first and secondswitching elements Q1 and Q2, thereby changing an energy quantity sentto the secondary side.

The first and second secondary windings S1 and S2 are coupled with eachother at the same polarity. In an ON period of the second switchingelement Q2, energy from the first secondary winding S1 is outputted asthe first output voltage Vo1. During this period, energy from the secondsecondary winding S2 is outputted as the second output voltage V02,which is substantially equal to a level of Vo1×N3/N2.

In practice, however, voltages generated by the first and secondsecondary windings S1 and S2 are higher than the first and second outputvoltages Vo1 and Vo2 each by a forward voltage drop Vf of the diodes D1and D2. Accordingly, a change in Vf due to a change in load on eachoutput worsens a cross regulation. In a power source apparatus withvariable output voltages, a change in one output voltage results inproportionally changing the other output voltage. This makes itimpossible to directly provide a plurality of outputs from windings.

FIG. 3 is a circuit diagram illustrating a configuration of amultiple-output switching power source apparatus according to anotherrelated art. This multiple-output switching power source apparatusemploys, instead of the second rectifying-smoothing circuit illustratedin FIG. 1, a regulator 12 such as a dropper or a step-down chopper, togenerate a second output voltage V02 from a first output voltage Vo1 soas to stabilize the outputs. This multiple-output switching power sourceapparatus may solve the cross regulation problem between two outputs.The regulator 12, however, increases a loss and additional parts such asswitching elements, choke coils, and control ICs increase costs andpackaging spaces. In addition, the switching regulator such as astep-down chopper unavoidably generates noise.

A multiple-output switching power source apparatus disclosed in JapaneseUnexamined Patent Application Publication No. 2003-259644 proposes aswitching converter circuit that stabilizes two kinds of voltage with asingle converter. This switching converter circuit employs a secondswitching element as an active snubber to control ON/OFF of a firstswitching element and stabilize a first output. During an OFF period ofthe first switching element, the circuit controls ON/OFF of the secondswitching element to stabilize a second output. This switching convertercircuit may stabilize two kinds of output with a single converter. Thiscircuit, however, must have two secondary windings because a secondarywinding to provide the first output must have an opposite polarity withrespect to a secondary winding that provides the second output.

DISCLOSURE OF INVENTION

As explained above, the multiple-output switching power source apparatusaccording to the related art has the problem that, if a change occurs inload on each output, a cross regulation worsens. The power sourceapparatus with variable output voltages has the problem that windings ofthe apparatus are unable to directly provide the plurality of outputs.The related art that employs a regulator on the secondary side to solvethe problem of cross regulation has the problems that the regulatorincreases a loss, additional parts increase costs and packaging spaces,and the regulator generates noise. The switching converter circuitdisclosed in the patent document 1 carries out no current resonance, andtherefore, there is an occasion that the converter is switched while acurrent is passing through rectifying diode on the secondary side. Thisraises a problem of generating noise.

The present invention provides a multiple-output switching power sourceapparatus capable of stabilizing a plurality of outputs irrespective ofa change in load.

Means to Solve the Problems

To solve the above-mentioned problems, a first technical aspect of thepresent invention provides a multiple-output switching power sourceapparatus having first and second switching elements connected in seriesbetween electrodes of a DC power source; a first series resonant circuitconnected in parallel with the first or second switching element andincluding a primary winding of a first transformer and a first currentresonant capacitor; a first rectifying-smoothing circuit configured torectify and smooth a voltage generated by a secondary winding of thefirst transformer in an ON period of the first or second switchingelement and provide a first output voltage; a second series resonantcircuit connected in parallel with the first or second switching elementand including a primary winding of a second transformer and a secondcurrent resonant circuit; a second rectifying-smoothing circuitconfigured to rectify and smooth a voltage generated by a secondarywinding of the second transformer in the ON period of the first orsecond switching element and provide a second output voltage; a controlcircuit configured to control an ON period of the first switchingelement according to the first output voltage and an ON period of thesecond switching element according to the second output voltage; and alimiting circuit configured to limit the ON period of the firstswitching element if the second output voltage exceeds a predeterminedvoltage.

A second technical aspect of the present invention provides amultiple-output switching power source apparatus having first and secondswitching elements connected in series between electrodes of a DC powersource; a first series resonant circuit connected in parallel with thefirst or second switching element and including a primary winding of afirst transformer and a first current resonant capacitor; a firstrectifying-smoothing circuit configured to rectify and smooth a voltagegenerated by a secondary winding of the first transformer in an ONperiod of the first or second switching element and provide a firstoutput voltage; a second series resonant circuit connected in parallelwith the first or second switching element and including a primarywinding of a second transformer and a second current resonant circuit; asecond rectifying-smoothing circuit configured to rectify and smooth avoltage generated by a secondary winding of the second transformer inthe ON period of the first or second switching element and provide asecond output voltage; a control circuit configured to control an ONperiod of the second switching element according to the first outputvoltage and an ON period of the first switching element according to thesecond output voltage; and a limiting circuit configured to limit the ONperiod of the first switching element if the second output voltageexceeds a predetermined voltage.

A third technical aspect of the present invention provides amultiple-output switching power source apparatus having first and secondswitching elements connected in series between electrodes of a DC powersource; a first series resonant circuit connected in parallel with thefirst or second switching element and including a primary winding of afirst transformer and a first current resonant capacitor; a firstrectifying-smoothing circuit configured to rectify and smooth a voltagegenerated by a secondary winding of the first transformer in an ONperiod of the first or second switching element and provide a firstoutput voltage; a second series resonant circuit connected in parallelwith the first or second switching element and including a primarywinding of a second transformer and a second current resonant circuit; asecond rectifying-smoothing circuit configured to rectify and smooth avoltage generated by a secondary winding of the second transformer inthe ON period of the first or second switching element and provide asecond output voltage; and a control circuit configured to control an ONduty of the first switching element according to the first outputvoltage, and according to the second output voltage, a switchingfrequency at which the first and second switching elements arealternately turned on/off.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram illustrating a configuration of amultiple-output switching power source apparatus according to a relatedart.

FIG. 2 is a waveform diagram illustrating operation of themultiple-output switching power source apparatus according to therelated art.

FIG. 3 is a circuit diagram illustrating a configuration of amultiple-output switching power source apparatus according to anotherrelated art.

FIG. 4 is a circuit diagram illustrating a configuration of amultiple-output switching power source apparatus according to Embodiment1 of the present invention.

FIG. 5 is a waveform diagram when an output Vo2 of the multiple-outputswitching power source apparatus according to Embodiment 1 of thepresent invention operates under heavy load.

FIG. 6 is a waveform diagram when the output Vo2 of the multiple-outputswitching power source apparatus according to Embodiment 1 of thepresent invention operates under light load.

FIG. 7 is a circuit diagram illustrating an example of a control circuitof the multiple-output switching power source apparatus according toEmbodiment 1 of the present invention.

FIG. 8 is a waveform diagram illustrating operation of the controlcircuit example of the multiple-output switching power source apparatusaccording to Embodiment 1 of the present invention.

FIG. 9 is a circuit diagram illustrating an example of a control circuitof a multiple-output switching power source apparatus according toEmbodiment 2 of the present invention.

FIG. 10 is a waveform diagram illustrating operation of the controlcircuit example of the multiple-output switching power source apparatusaccording to Embodiment 2 of the present invention.

FIG. 11 is a circuit diagram illustrating an example of a controlcircuit of a multiple-output switching power source apparatus accordingto Embodiment 3 of the present invention.

FIG. 12 is a waveform diagram illustrating operation of the controlcircuit example of the multiple-output switching power source apparatusaccording to Embodiment 3 of the present invention.

BEST MODE OF IMPLEMENTING INVENTION

Multiple-output switching power source apparatuses according toembodiments of the present invention will be explained in detail withreference to the drawings. The same or corresponding parts as those ofthe multiple-output switching power source apparatuses explained inBACKGROUND TECHNOLOGY will be explained with the use of the samereference marks as those used in BACKGROUND TECHNOLOGY.

Embodiment 1

FIG. 4 is a circuit diagram illustrating a configuration of amultiple-output switching power source apparatus according to Embodiment1 of the present invention. This multiple-output switching power sourceapparatus has first and second transformers T1 a and T2 a. On theprimary side of the first and second transformers T1 a and T2 a, thereare arranged a full-wave rectifying circuit 2 to rectify an AC voltagefrom a commercial power source 1, a smoothing capacitor C3 connectedbetween output terminals of the full-wave rectifying circuit 2, tosmooth an output from the full-wave rectifying circuit 2, first andsecond switching elements Q1 and Q2 made of, for example, MOSFETsconnected in series between both ends of the smoothing capacitor C3, toreceive a voltage across the smoothing capacitor C3 as a DC inputvoltage Vin, a control circuit 10 a to control ON/OFF of the first andsecond switching elements Q1 and Q2, a voltage resonant capacitor Crvconnected in parallel with the second switching element Q2, and firstand second series resonant circuits connected to both ends of thevoltage resonant capacitor Crv.

The first series resonant circuit consists of a primary winding P1 (thenumber of turns of N1 a) of the first transformer Tla, a first resonantreactor Lr1, and a first current resonant capacitor Cri that areconnected in series. The first resonant reactor Lr1 is, for example, aleakage inductance between the primary and secondary sides of the firsttransformer Tla.

The second series resonant circuit consists of a primary winding P2 (thenumber of turns of N1 b) of the second transformer T2 a, a secondresonant reactor Lr2, and a second current resonant capacitor Cri2 thatare connected in series. The second resonant reactor Lr2 is, forexample, a leakage inductance between the primary and secondary sides ofthe second transformer T2 a.

On the secondary side of the first transformer Tla, there is arranged afirst rectifying-smoothing circuit connected to a secondary winding S1(the number of turns of N2 a) wound to generate a voltage whose phase isopposite to the phase of a voltage generated by the primary winding P1of the first transformer Tla. The first rectifying-smoothing circuit hasa diode D1 and a smoothing capacitor C1. An anode of the diode D1 isconnected to a first end of the secondary winding S1 and a cathodethereof is connected to a first output terminal. The smoothing capacitorC1 is connected between the cathode of the diode D1 (the first outputterminal) and a second end of the secondary winding S1 (a terminal GND).The first rectifying-smoothing circuit rectifies and smoothes a voltageinduced by the secondary winding S1 of the first transformer Tla andoutputs a first output voltage Vo1 from the first output terminal.

On the secondary side of the second transformer T2 a, there is arrangeda second rectifying-smoothing circuit connected to a secondary windingS2 (the number of turns of N2 b) wound to generate a voltage whose phaseis opposite to the phase of a voltage generated by the primary windingP2 of the second transformer T2 a. The second rectifying-smoothingcircuit consists of a diode D2 and a smoothing capacitor C2. An anode ofthe diode D2 is connected to a first end of the secondary winding S2 anda cathode thereof is connected to a second output terminal. Thesmoothing capacitor C2 is connected between the cathode of the diode D2(the second output terminal) and a second end of the secondary windingS2 (a terminal GND). The second rectifying-smoothing circuit rectifiesand smoothes a voltage induced by the secondary winding S2 of the secondtransformer T2 a and outputs a second output voltage V02 from the secondoutput terminal.

The multiple-output switching power source apparatus also has a feedbackcircuit 5 to feed a voltage generated on the secondary side of the firsttransformer Tla back to the primary side and a feedback circuit 6 tofeed a voltage generated on the secondary side of the second transformerT2 a back to the primary side. The feedback circuit 5 compares the firstoutput voltage Vo1 outputted to the first output terminal with apredetermined reference voltage and feeds an error voltage as a firstvoltage error signal back to the control circuit 10 a on the primaryside. The feedback circuit 6 compares the second output voltage V02outputted to the second output terminal with a predetermined referencevoltage and feeds an error voltage as a second voltage error signal backto the control circuit 10 a on the primary side.

According to the first voltage error signal from the feedback circuit 5and the second voltage error signal from the feedback circuit 6, thecontrol circuit 10 a alternately turns on/off the first and secondswitching elements Q1 and Q2, to thereby conduct PWM control that keepsthe first and second output voltages Vo1 and Vo2 constant. In this case,gates of the first and second switching elements Q1 and Q2 receive, ascontrol signals, voltages involving a dead time of about severalhundreds of nanoseconds. This allows the first and second switchingelements Q1 and Q2 to alternately turn on/off without ON periods of theswitching elements Q1 and Q2 overlapping each other.

More precisely, the control circuit 10 a controls the ON period of thesecond switching element Q2 according to the second voltage error signalprovided by the feedback circuit 6 and controls the ON period of thefirst switching element Q1 according to the first voltage error signalprovided by the feedback circuit 5.

The control circuit 10 a may control the ON period of the secondswitching element Q2 according to the first voltage error signal that isbased on the output voltage Vo1 and may control the ON period of thefirst switching element Q1 according to the second voltage error signalthat is based on the output voltage V02.

Operation of the multiple-output switching power source apparatusaccording to Embodiment 1 of the present invention having such aconfiguration will be explained with reference to a waveform diagramillustrated in FIG. 5 in which the output Vo2 is under heavy load and awaveform diagram illustrated in FIG. 6 in which the output Vo2 is underlight load.

In FIGS. 5 and 6, VQ2 ds is a drain-source voltage of the secondswitching element Q2, Icri is a current passing through the firstcurrent resonant capacitor Cri, Vcri is a voltage across the firstcurrent resonant capacitor Cri, ID1 is a current passing through thediode D1, Icri2 is a current passing through the second current resonantcapacitor Cri2, Vcri2 is a voltage across the second current resonantcapacitor Cri2, and ID2 is a current passing through the diode D2.

In an ON period (time t1 to t2) of the first switching element Q1, thefirst and second series resonant circuits receive the input voltage Vinand carry out a resonant operation to pass excitation currents throughthe primary windings P1 and P2 and charge the first and second currentresonant capacitors Cri and Cri2.

When the first switching element Q1 turns off and the second switchingelement Q2 turns on (from time t2 to t3), voltages of the first andsecond current resonant capacitors Cri and Cri2 are applied to theprimary windings P1 and P2 of the first and second transformers T1 a andT2 a and the first and second resonant reactors Lr1 and Lr2 and firstand second current resonant capacitors Cri and Cri2 pass resonantcurrents, which are transferred to the secondary side. As a result, thesecondary windings S1 and S2 induce voltages, which are rectified by thediodes D1 and D2 and are supplied as the first and second outputvoltages Vo1 and Vo2 from the first and second output terminals.

In this way, in the multiple-output switching power source apparatus,the first and second series resonant circuits similarly operate. If theleakage inductance Lr1 is decreased and the capacitance of the firstcurrent resonant capacitor Cri is increased in the first series resonantcircuit, and in the second series resonant circuit, the leakageinductance Lr2 is increased and the capacitance of the second currentresonant capacitor Cri2 is decreased, power supplied to the outputvoltage Vo1 and power supplied to the output voltage V02 can be changed.

When the first switching element Q1 has an ON duty of Don1 and the firstand second switching elements Q1 and Q2 are alternately turned on/off,an average of the voltages of the first and second current resonancecapacitors Cri and Cri2 will be a level of Vin×Don1.

In the first series resonant circuit, the capacitance of the firstcurrent resonant capacitor Cri is large, and therefore, has a smallamplitude. Since the leakage inductance Lr1 is small, an impedancebetween the primary and secondary sides of the first transformer Tla issmall. A voltage generated by the secondary winding S1 of the firsttransformer T1 a in an ON period of the second switching element Q2 issubstantially equal to a product obtained by multiplying the voltage ofthe first current resonant capacitor Cri by a turn ratio. Accordingly,the output voltage Vo1 is controllable by adjusting the ON duty of thefirst switching element Q1.

On the other hand, the second current resonant capacitor Cri2 in thesecond series resonant circuit has small capacitance, and therefore, thesecond current resonance capacitor Cri2 has a large amplitude. Since theleakage inductance Lr2 is large and the voltage of the second currentresonant capacitor Cri2 is limited by the leakage inductance Lr2, avoltage generated by the secondary winding S2 of the second transformerT2 a is not equal to a product obtained by multiplying the voltage ofthe second current resonant capacitor Cri2 by a turn ratio.

Due to this, a power supplied to the secondary side should be adjustedin order to adjust the voltage amplitude of the second current resonantcapacitor Cri2. The voltage amplitude of the second current resonantcapacitor Cri2 is adjustable by adjusting a switching frequency or byadjusting the ON width of the first switching element Q1. Namely,adjusting the ON duty of the first switching element Q1 results incontrolling the output voltage Vo1 and adjusting the switching frequencyof the first and second switching elements Q1 and Q2 results incontrolling the output voltage Vo2.

Concrete Example of Control Circuit

A concrete example of the control circuit of the multiple-outputswitching power source apparatus illustrated in FIG. 4 will be explainedwith reference to FIGS. 7 and 8. FIG. 7 is a circuit diagramillustrating an example of the control circuit 10 a illustrated in FIG.4. FIG. 8 is a waveform diagram illustrating operation of the controlcircuit illustrated in FIG. 7.

In the control circuit 10 a illustrated in FIG. 7, connected between areference power source Vref and the ground are a first series circuitincluding a resistor R8 and a photocoupler PC1 and a second seriescircuit including resistors R12 and R13 and a photocoupler PC2. Thephotocoupler PC1 is included in the feedback circuit 5 and transfers afeedback signal from the output voltage Vo1 to the control circuit 10 a.The photocoupler PC2 is included in the feedback circuit 6 and transfersa feedback signal from the output voltage V02 to the control circuit 10a.

A plus terminal (depicted by “+”) of a comparator CMP1 is connected to aconnection point between the resistor R8 and the photocoupler PC1 and aminus terminal (depicted by “−”) of the comparator CMP1 is connected toa first end of a capacitor C10, an anode of a diode D10, and a first endof a resistor R10. A second end of the capacitor C10 is grounded. Acathode of the diode D10 and a second end of the resistor R10 areconnected to an output end of a delay circuit 13 and an input end of alevel shift circuit 17.

The comparator CMP1 compares a voltage at the plus terminal with avoltage at the minus terminal and outputs a comparison result to a resetterminal R of an RS flip-flop circuit 11 (hereinafter referred to as“RSF/F 11”).

A plus terminal (depicted by “+”) of a comparator CMP2 is connected to aconnection point between the resistors R12 and R13 and a minus terminal(depicted by “−”) of the comparator CMP2 is connected to a first end ofa capacitor C11, an anode of a diode D11, and a first end of a resistorR11. A second end of the capacitor C11 is grounded. A cathode of thediode D11 and a second end of the resistor R11 are connected to anoutput end of a delay circuit 15 and a gate of the switching element Q2.

The comparator CMP2 compares a voltage at the plus terminal with avoltage at the minus terminal and outputs a comparison result to a setterminal S of the RSF/F 11.

The delay circuit 13 delays an output Q of the RSF/F 11 by apredetermined time to prevent the first and second switching elements Q1and Q2 from simultaneously turning on and is connected through the levelshift circuit 17 to a gate terminal of the first switching element Q1.The delay circuit 15 delays an inverted output Q1 of the RSF/F 11 by apredetermined time to prevent the first and second switching elements Q1and Q2 from simultaneously turning on and is connected to the gateterminal of the switching element Q2.

Operation of the control circuit illustrated in FIG. 7 having theabove-mentioned configuration will be explained with reference to thewaveform diagram of FIG. 8.

When the output Q of the RSF/F 11 is high (time t0), a gate drive signalis applied through the delay circuit 13 and level shift circuit 17 tothe gate of the first switching element Q1, to turn on the firstswitching element Q1. The high-level output Q of the RSF/F 11 graduallycharges the capacitor C10 through the resistor R10.

When the voltage of the capacitor C10 reaches a voltage at the plusterminal of the comparator CMP1 (time t1), the output of the comparatorCMP1 inverts to provide the reset terminal R (negative logic in thisembodiment) of the RSF/F 11 with a low-level signal.

This results in inverting the output of the RSF/F 11, so that the outputQ becomes low and the inverted output Q1 high. Then, the gate voltage ofthe switching element Q1 drops to turn off the first switching elementQ1 and discharge the capacitor C10 to drop the voltage of the capacitorC10.

Since the inverted output Q1 of the RSF/F 11 is high, a gate drivesignal is applied through the delay circuit 15 to the second switchingelement Q2 to turn on the second switching element Q2 and graduallycharge the capacitor C11 through the resistor R11 (time t1 to t2).

When the voltage of the capacitor C11 reaches a voltage at the plusterminal of the comparator CMP2 (time t2), the output of the comparatorCMP2 inverts to provide the set terminal S (negative logic in thepresent embodiment) of the RSF/F 11 with a low level signal. Thisinverts the output of the RSF/F 11, so that the inverted output Q1becomes low and the output Q high. Then, the gate voltage of the secondswitching element Q2 drops to turn off the second switching element Q2and the capacitor C10 discharges to drop the voltage thereof.

Through the delay circuit 13 and level shift circuit 17, a gate drivesignal is applied to the first switching element Q1 to turn on the firstswitching element Q1. These operations are repeated to alternately turnon/off the first and second switching elements Q1 and Q2.

The outputs Vo1 and Vo2 are controlled by changing voltages at thephotocouplers PC1 and PC2 through the feedback circuits 5 and 6 tochange the ON widths, ON duties, and frequency of the first and secondswitching elements Q1 and Q2.

If load on the output voltage V02 becomes lighter, a feedback signalfrom the feedback circuit 6 lowers the voltage of the photocoupler PC2to shorten the ON width of the second switching element Q2.

This increases the ON duty of the first switching element Q1 to increasethe output voltage Vo1. Then, a feedback signal from the feedbackcircuit 5 lowers the voltage of the photocoupler PC1 to shorten the ONwidth of the first switching element Q1, thereby controlling the ON dutyof the first switching element Q1.

These operations change the switching frequency in such a way as tostabilize the output Vo2. As mentioned above, the multiple-outputswitching power source apparatus of Embodiment 1 illustrated in FIG. 4employs a pair of half-bridge converters to stabilize the two outputvoltages Vo1 and Vo2.

Embodiment 2

According to the multiple-output switching power source apparatusillustrated in FIG. 4, if the output voltage V02 first rises at thestart of the apparatus, the feedback circuit 6 transfers an error signalrelated to the output voltage V02 to the control circuit 10 a on theprimary side and the control circuit 10 a tries to shorten the ON widthof the second switching element Q2.

The output voltage Vo1, however, is equal to or lower than a setvoltage, and therefore, there is no feedback signal from the feedbackcircuit 5. Accordingly, the ON width of the first switching element Q1is maximally opened. It is, therefore, impossible to restrict thevoltage amplitude of the second current resonant capacitor Cri2 and theoutput voltage V02 is uncontrollable even if the ON width of the secondswitching element Q2 is narrowed. Since the ON width of the secondswitching element Q2 is narrowed, the ON duty of the first switchingelement Q1 becomes larger to increase an average of the second currentresonant capacitor Cri2 and further increase the output voltage V02.

A similar problem arises if the output voltage Vo1 decreases due to, forexample, overload. In this way, the multiple-output switching powersource apparatus illustrated in FIG. 4 sharply increases the outputvoltage V02 depending on load balance at the start of the apparatus orunder overload.

To cope with this, the multiple-output switching power source apparatusaccording to Embodiment 2 prevents two outputs from leaping at the startof the apparatus or under overload.

FIG. 9 is a circuit diagram illustrating an example of a control circuitin the multiple-output switching power source apparatus according toEmbodiment 2 of the present invention. Compared with the control circuitin the multiple-output switching power source apparatus illustrated inFIG. 4, the multiple-output switching power source apparatus ofEmbodiment 2 illustrated in FIG. 9 additionally has an operationalamplifier OP1, a diode D12, and resistors R14 and R15, to form alimiting circuit of the present invention.

A non-inverting terminal (as depicted by “+”) of the operationalamplifier OP1 is connected to a connection point between a collector ofa photocoupler PC2 and a resistor R13 and an inverting terminal (asdepicted by “−”) of the operational amplifier OP1 is connected to afirst end of the resistor R14 and a first end of the resistor R15. Asecond end of the resistor R15 is grounded. An output end of theoperational amplifier OP1 is connected to a cathode of the diode D12 anda second end of the resistor R14. An output voltage of the operationalamplifier OP1 is divided by the resistors R14 and R15 and is fed back tothe inverting terminal of the operational amplifier OP1. An anode of thediode D12 is connected to a plus terminal (depicted by “+”) of acomparator CMP1 and a connection point between a collector of aphotocoupler PC1 and a resistor R8.

FIG. 10 is a waveform diagram illustrating a voltage of the photocouplerPC2 and a voltage of the operational amplifier OP1 in the controlcircuit of FIG. 9. With reference to FIG. 10, operation of the controlcircuit of FIG. 9 will be explained. In FIG. 10, the photocoupler PC2has a steady operation range from a voltage level LV1 to a voltage levelLV2.

An output of the operational amplifier OP1 is restricted in its upperlimit by a power source voltage of the operational amplifier OP1. Theinverting terminal of the operational amplifier OP1 receives a voltagethrough the resistors R14 and R15 that divide the upper limit of theoutput voltage of the operational amplifier OP1. Resistance values ofthe resistors R14 and R15 are preset so that the divided voltage fromthe resistors R14 and R15 is low, about several hundreds of millivoltsto one volt.

If there is no feedback signal from the feedback circuit 6 before theoutput voltage V02 rises (before time t21), the voltage of thephotocoupler PC2 is high and a voltage at the non-inverting terminal ofthe operational amplifier OP1 is sufficiently higher than a voltage atthe inverting terminal thereof. As results, the output of theoperational amplifier OP1 has a level of an output upper limit LV3.

When the output voltage V02 rises and the feedback circuit 6 transfers afeedback signal, the voltage of the photocoupler PC2 graduallydecreases. When the output voltage V02 exceeds a set voltage, thefeedback signal increases and the voltage of the photocoupler PC2further decreases.

When the voltage of the photocoupler PC2 becomes equal to or lower thanthe voltage at the inverting terminal of the operational amplifier OP1,the output voltage of the operational amplifier OP1 decreases relativeto the voltage of the photocoupler PC2 (from time t21) and theoperational amplifier OP1 functions to equalize the voltages at theinverting and non-inverting terminals.

When the voltage of the photocoupler PC2 further decreases and theoutput voltage of the operational amplifier OP1 becomes equal to orlower than the voltage at the plus terminal of the comparator CMP1 (timet22), the diode D12 turns on to pass a current through a path extendingalong Vref, R8, D12, the output terminal of OP1, and the ground. Asresults, the voltage at the plus terminal of the comparator CMP1decreases from the voltage level LV4 (at this time, the photocoupler PC2is at the voltage level LV2) and reaches the voltage level LV5 at time23. At this time, the voltage at the inverting terminal of theoperational amplifier OP1 is nearly zero. Consequently, the comparatorCMP1 acts to reduce the ON width of the first switching element Q1.

During a steady operation, the voltage of the photocoupler PC2 iscontrolled to be within the steady operation range illustrated in FIG.10. If the output voltage V02 abnormally increases at the start of theapparatus or the like, not only the ON width of the second switchingelement Q2 but also the ON width of the first switching element Q1 arenarrowed to control a switching frequency, thereby avoiding the abnormalincrease of the output voltage V02.

According to this embodiment, the limiting circuit refers to an errorvoltage obtained by comparing the second output voltage with a referencevoltage, to see if the second output voltage is equal to or larger thana predetermined voltage. Accordingly, the limiting circuit can beincorporated in the control circuit, to realize a compact device.

Embodiment 3

FIG. 11 is a circuit diagram illustrating an example of a controlcircuit of a multiple-output switching power source apparatus accordingto Embodiment 3 of the present invention.

In FIG. 11, connected between a reference power source Vref and theground are a first series circuit including a resistor R8 and aphotocoupler PC1 and a second series circuit including a photocouplerPC2, a resistor R17, and a capacitor C12. The photocoupler PC2 isconnected in parallel with a resistor R16.

A connection point between the resistor R17 and the capacitor C12 isconnected to a minus terminal (depicted by “−”) of a comparator CMP1, aminus terminal (depicted by “−”) of a comparator CMP2, and an anode of adiode D13. A cathode of the diode D13 is connected to an output end ofthe comparator CMP2 and a set terminal S of an RSF/F 11. A plus terminal(depicted by “+”) of the comparator CMP2 is connected to a referencepower source Vref2.

A plus terminal (depicted by “+”) of the comparator CMP1 is connected toa connection point between the resistor R8 and the photocoupler PC1 andan output end of the comparator CMP1 is connected to a reset terminal Rof the RSF/F 11.

The remaining configuration is the same as that illustrated in FIG. 7,and therefore, the same parts are represented with the same referencemarks and their explanations are omitted.

FIG. 12 is a waveform diagram illustrating operation of the controlcircuit example of the multiple-output switching power source apparatusaccording to Embodiment 3 of the present invention. Operation of themultiple-output switching power source apparatus of Embodiment 3 will beexplained with reference to FIG. 12.

At time t0, an output Q of the RSF/F 11 is initially high and a voltageof the capacitor C12 is initially 0 V. The voltage of the capacitor C12gradually increases as the capacitor C12 is charged by the referencepower source Vref through the photocoupler PC2 and resistors R16 andR17.

At time t1, the voltage of the capacitor C12 reaches the voltage of thephotocoupler PC1 and the output of the comparator CMP1 inverts to applya low-level signal to the reset terminal R (negative logic in thisembodiment) of the RSF/F 11. As results, the output Q of the RSF/F 11becomes low and an inverted output Q1 thereof becomes high.

Thereafter, the voltage of the capacitor C12 further increases, and attime t2, reaches the reference power source voltage Vref2. Then, theoutput of the comparator CMP2 inverts to provide the set terminal S ofthe RSF/F 11 with a low-level signal. This makes the inverted output Q1of the RSF/F 11 low and the output Q thereof high. At the same time, thevoltage of the capacitor C12 is discharged through the diode D13 andreturns to the initial state of 0 V. These operations are repeated toalternately turn on/off first and second switching elements Q1 and Q2.

When load on the output voltage V02 becomes lighter, a feedback signalfrom a feedback circuit 6 increases to increase a current passing to thephotocoupler PC2. Then, the voltage of the capacitor C12 becomes steeperto shorten the ON periods of the first and second switching elements Q1and Q2.

Since a control circuit of this embodiment uses the output voltage V02to control the ON period of the first switching element Q1 and theswitching frequency of the second switching element Q2, the outputvoltage V02 is stabilized.

The output voltage Vo1 is controlled like the control circuit of therelated art. Namely, the voltage of the photocoupler PC1 is changed tocontrol the ON duty of the first switching element Q1. When the outputvoltage V02 reaches a predetermined voltage to provide a feedback signalfrom a feedback circuit 6 at the start of the apparatus, the controlcircuit narrows the ON widths of the second and first switching elementsQ2 and Q1, to avoid the output voltage V02 from abnormally increasing.

When load on the output voltage V02 varies, the frequency of the firstand second switching elements Q1 and Q2 may change and the ON dutiesthereof may not change. Accordingly, the output voltage Vo1 will be lessaffected. Compared with the related art, the embodiment is advantageousbecause it improves response.

The present invention is not limited to the above-mentioned Embodiments1 to 3. According to Embodiments 1 to 3, the second series resonantcircuit is connected in parallel with the voltage resonant capacitorCrv. For example, the second series resonant circuit may be connected inparallel with the primary winding P1 of the first transformer Tla, orthe secondary winding S1 thereof, or the tertiary winding thereof. Thisis equivalent to connecting the second series resonant circuit inparallel with the first or second switching element and provides thesame effect.

EFFECT OF INVENTION

According to the first and second technical aspects of the presentinvention, the limiting circuit is arranged to limit the ON period ofthe first switching element if the second output voltage exceeds apredetermined voltage. Even if the first output voltage does not reach apredetermined voltage at the start of the apparatus or a restart thereofafter an activation of an overcurrent protection circuit, the ON widthof the first switching element is limited if the second output voltageexceeds the predetermined voltage, to suppress a voltage increase of thefirst current resonant capacitor, thereby avoiding an abnormal increaseof the second output voltage.

According to the third technical aspect of the present invention, thelimiting circuit controls the ON duty of the first switching elementaccording to the first output voltage and controls the switchingfrequency of the first and second switching elements according to thesecond output voltage. Accordingly, the limiting circuit operates likethe invention of claim 1. According to the second output voltage, theswitching frequency at which the first and second switching elements arealternately turned on/off is controlled. Even if the first outputvoltage does not reach a predetermined voltage at the start of theapparatus or a restart thereof after an activation of an overcurrentprotection circuit, the ON width of the first switching element islimited if the second output voltage exceeds a predetermined voltage, tosuppress a voltage increase of the first current resonant capacitor,thereby avoiding an abnormal increase in the second output voltage.

(United States Designation)

In connection with United States designation, this application claimsbenefit of priority under 35USC §119 to Japanese Patent Application No.2007-050207 filed on Feb. 28, 2007, the entire contents of which areincorporated by reference herein.

1. A multiple-output switching power source apparatus comprising: firstand second switching elements connected in series between electrodes ofa DC power source; a first series resonant circuit being connected inparallel with the first or second switching element and including aprimary winding of a first transformer and a first current resonantcapacitor; a first rectifying-smoothing circuit configured to rectifyand smooth a voltage generated by a secondary winding of the firsttransformer in an ON period of the first or second switching element andprovide a first output voltage; a second series resonant circuit beingconnected in parallel with the first or second switching element andincluding a primary winding of a second transformer and a second currentresonant circuit; a second rectifying-smoothing circuit configured torectify and smooth a voltage generated by a secondary winding of thesecond transformer in the ON period of the first or second switchingelement and provide a second output voltage; a control circuitconfigured to control an ON period of the first switching elementaccording to the first output voltage and an ON period of the secondswitching element according to the second output voltage; and a limitingcircuit configured to limit the ON period of the first switching elementif the second output voltage exceeds a predetermined voltage.
 2. Amultiple-output switching power source apparatus comprising; first andsecond switching elements connected in series between electrodes of a DCpower source; a first series resonant circuit being connected inparallel with the first or second switching element and including aprimary winding of a first transformer and a first current resonantcapacitor; a first rectifying-smoothing circuit configured to rectifyand smooth a voltage generated by a secondary winding of the firsttransformer in an ON period of the first or second switching element andprovide a first output voltage; a second series resonant circuit beingconnected in parallel with the first or second switching element andincluding a primary winding of a second transformer and a second currentresonant circuit; a second rectifying-smoothing circuit configured torectify and smooth a voltage generated by a secondary winding of thesecond transformer in the ON period of the first or second switchingelement and provide a second output voltage; a control circuitconfigured to control an ON period of the second switching elementaccording to the first output voltage and an ON period of the firstswitching element according to the second output voltage; and a limitingcircuit configured to limit the ON period of the first switching elementif the second output voltage exceeds a predetermined voltage.
 3. Themultiple-output switching power source apparatus according to claim 1,wherein the limiting circuit limits the ON period of the first switchingelement if an error voltage between the second output voltage and areference voltage is equal to or larger than a predetermined value. 4.The multiple-output switching power source apparatus according to claim2, wherein the limiting circuit limits the ON period of the firstswitching element if an error voltage between the second output voltageand a reference voltage is equal to or larger than a predeterminedvalue.
 5. A multiple-output switching power source apparatus comprising:first and second switching elements connected in series betweenelectrodes of a DC power source; a first series resonant circuit beingconnected in parallel with the first or second switching element andincluding a primary winding of a first transformer and a first currentresonant capacitor; a first rectifying-smoothing circuit configured torectify and smooth a voltage generated by a secondary winding of thefirst transformer in an ON period of the first or second switchingelement and provide a first output voltage; a second series resonantcircuit being connected in parallel with the first or second switchingelement and including a primary winding of a second transformer and asecond current resonant circuit; a second rectifying-smoothing circuitconfigured to rectify and smooth a voltage generated by a secondarywinding of the second transformer in the ON period of the first orsecond switching element and provide a second output voltage; and acontrol circuit configured to control an ON duty of the first switchingelement according to the first output voltage, and according to thesecond output voltage, a switching frequency at which the first andsecond switching elements are alternately turned on/off.
 6. Themultiple-output switching power source apparatus according to claim 1,wherein the second series resonant circuit is connected in parallel withthe primary, secondary, or tertiary winding of the first transformer. 7.The multiple-output switching power source apparatus according to claim2, wherein the second series resonant circuit is connected in parallelwith the primary, secondary, or tertiary winding of the firsttransformer.
 8. The multiple-output switching power source apparatusaccording to claim 5, wherein the second series resonant circuit isconnected in parallel with the primary, secondary, or tertiary windingof the first transformer.